Vertical mixing and weak stratification over zebra mussel colonies in western Lake Erie

Zebra mussels (Dreissena polymorpha) are an invasive species that have been implicated in the reduction of algae stocks in the near-shore environment of western Lake Erie. To determine their basin-wide effects, we applied a twodimensional hydrodynamic and water-quality model for 1994. The model accurately reproduced lake-wide hydrodynamics and water quality. When modeled as true benthic organisms (resting on the bottom), the dreissenids grazed 53% of the western basin May through September net algal growth. This grazing resulted in a ,0.1-mg L21 reduction in the pelagic algae concentration relative to the case without dreissenids. In comparison, dreissenids grazed 77% western basin net algal growth when the lake was modeled as a fully mixed water column. We found that the biomass grazed was governed by a balance between the timescales of vertical wind-induced mixing and benthic grazing. During calm conditions, weak diurnal stratification (,1uC between surface and bottom waters) was sufficient to suppress vertical mixing, when the mean daily wind speed 4 m above the lake surface (U4) was ,6 m s21. These conditions allowed a concentration boundary layer ,1 m thick to form, accounting for the reduced grazing effect relative to the fully mixed case. Entrainment of the concentration boundary layer occurred for U4 .6 m s21 (associated with the lake’s characteristic 10-d storm cycle) facilitating algae supply to the benthos. We formulated the mean daily biomass grazed in terms of the dreissenid areal pumping rate (a) and U4 and found that because typically U4 is ,6 m s21, the western basin is weakly stratified thermally and a concentration boundary layer forms when U4 ,3a or a .2 m3 m22 d21. The dynamics of both wind-mixing and thermal stratification must, therefore, be considered in mixing models applied to shallow weakly stratified lake basins. Zebra mussels (Dreissena polymorpha) successfully invaded the Laurentian Great Lakes in the mid–1980s. By the 1990s they had achieved an ecologically dominant status in the benthos of Lake Erie (e.g., Berkman et al. 1998). Their abundance and large areal filtering capacity have led to the mussels being implicated in the dramatic increase in water clarity observed since the late 1960s (e.g., Hebert et al. 1991). The establishment of the mussels was preceded by the1970s phosphorus load abatement programs (DePinto et al. 1986) and consequently delineation of the relative effects of these processes on Lake Erie water quality remains a challenge (Charlton 1994, Boegman et al. in press). The subsequent colonization by the quagga mussel (D. bugensis) further complicates the analysis. Observational studies have shown that the effect of dreissenid mussels is extreme in shallow near-shore environments with high mussel densities (Hecky et al. 2004). Dreissenid mussel densities as high as 2.5 3 105 mussels m22 have been observed in western Lake Erie and are among the largest reported for any freshwater mollusk (MacIsaac et al. 1992). When these densities are coupled with pumping rates estimated to be as high as 234 mL mussels21 h21 (Yu and Culver 1999), the potential filtering capacity is enormous (,60 m3 m22 h21). Potential filtering capacity (PFC) models have been applied to gauge the effects of dreissenid mussels on the algae stocks of the shallow and productive Lake Erie western basin. MacIsaac et al. (1992) argued that the western basin is usually well-mixed vertically, a seasonal thermocline does not form, and the temperature difference between surface and bottom waters is usually confined to #1uC. Under these conditions, they suggested that dreissenid mussels would have access to phytoplankton throughout 1 Corresponding author (email). 2 Retired.